Microwave oven defrost circuit

ABSTRACT

An energization circuit for a microwave oven provides for microwave oven operation in cook and defrost modes. A start switch in series with a current limiting resistor prevents high current surges during initial energization of a transformer used to energize a magnetron that produces microwave energy, and a relay operated switch effects a current bypass of such resistor after initial transformer and magnetron energization. The energization circuit includes a selector switch for coupling the circuit in defrost configuration that includes a motor-controlled multiple stage energization circuit for the magnetron, such circuit having an on and off time for reducing the average level of microwave energy within the microwave cooking cavity to achieve food defrosting without cooking the same. A current limiting resistor is sequentially connected in the multiple stage energization circuit for preventing high current surges as the transformer and magnetron are cyclically energized in the defrost mode.

LATCH DOOR STIRRER 5 TIMER United States Patent 1 1 1 1 3,842,233 Lamb Oct. 15, 1974 MICROWAVE OVEN DEFROST CIRCUIT Primary Examiner-J. V.r;Tlr)uhJe Assistant ExaminerHu ae er [75] Inventor' John Lamb Mansfield Ohm Attorney, Agent, or Firm Donnel ly, Maky, Renner &

[73] Assignee: The Tappan Company, Mansfield, 0m

Ohio

[57] ABSTRACT [22], Flled: May 1973 An energization circuit for a microwave oven provides [21] Appl. No.2 365,409 for microwave oven operation in cook and defrost 1, ,6 V i modes. A start switch in series with a current limiting resistor prevents high current surges during initial en- 52 US. Cl. .1 219/1055 ergizatio a P' used energize a magne [51} Int. Cl. 1105b 9/06 that Produces m'crowave energy and a relay. [58] Field 61 Search 219/1055 Grated sw'tch effws a Current bypass 0f Such ressmr MAW W after initial transformer and magnetron energization. Y I V V 7 'i i The energization circuit includes a selector switch for coupling the circuit in defrost configuration that in- [56] References Cited cludes a motor-controlled multiple stage energization UNITED STATES PATENTS circuit for the magnetron, such circuit having an on 3,398,371 8/1968 Lamb 219/1055 and Off time for reducing the average level of micro 3,470,942 10/1969 Fukada at 219/1055 X wave energy within the microwave cooking cavity to 3,569,656 3/1971 White et al. 219/1055 achieve food defrosting Without Cooking the Same- A 3,624,334 11/1971 Chapel] 1 219/1055 current limiting resistor is sequentially connected in 3,694,608 9/1972 Loubert 219/ 10.55 the multiple stage energization circuit for preventing high current surges as the transformer and magnetron are cyclically energized in the defrost mode.

6 Claims, 2 Drawing Figures 1 MICROWAVE OVEN DEFROST CIRCUIT This invention relates to a circuit for energizing a magnetron-or the like for operation of a microwave oven in cook and defrost modes and more particularly to such a circuit that effects control of the average energy level within the microwave cooking cavity.

A microwave oven provides a food cooking function by the production of microwaves or other similar electromagnetic energy within the oven cooking cavity, such energy effecting cooking of the food therein. In conventional microwave ovens a magnetron tube energized through a transformer coupling produces microwave energy to cook food within the oven cooking cavity in a very short time. Since the magnetron operates at a substantially constant level, it is difficult to control the energy level within the oven cooking cavity, one method of control being a cyclical turning on and off of the magnetron to achieve a reduced average energy level.

The large magnetizing current drawn by the transformer when starting poses a difiiculty in that it tends to saturate and subjects the other components in the energization circuit to damage, such as for example, by welding switch contacts together or unnecessarily burning-out thesafe'ty fuses in such circuit. Moreover, such high starting currents constitute a start-up shock to the magnetron which reduces the effective useful life thereof.

One method for avoiding such high current pulses during'starting has been to use a start switch in series connection with a current limiting resistor in the energization circuit for the magnetron to provide a low starting current therefor; and a surge relay or power relay having a small inherent delayed response is coupled across the energization circuit to operate a normally open switch for bypassing the resistor shortly after the magnetron is energized. lt has been found by using the start switch and current limiting resistor series combination that the peak current drawn by the magnetron during starting has been reduced from about 150 amps down to about 26 amps.

Accordingly, a primary object of the invention is to reducethe average energy output from an electric apparatus, such as a magnetron or the like.

Another object of the invention is to reduce switching current surges in the energization circuit for an electric apparatus, such as a magnetron or the like.

A further object of the invention is to reduce start-up shock to a magnetron produced by the high magnetizing currents thereof.

An additional object of the invention is to prolong the useful life of an electric apparatus, such as a magnetron or the like and the energization circuit therefor.

Still another object of the invention is to provide a circuit for effecting production in a microwave oven cooking cavity energy sufficient to defrost food without rapidly cooking the same.

Still a further object of the invention is to provide a circuit for a microwave oven to permit operation in cook and defrost modes.

These and other objects and advantages are realized in the instant invention which comprises an energization circuit for an electric apparatus, such as a magnetron and the like, that produces in response to an electrical input an electromagnetic energy output, such energization circuit including a multiple stage cycling 2 switch for effecting energization of the magnetron cyclically and at various current levels by coupling a current limiting resistor in the energization circuit briefly at the beginning of each such cycle, whereby the magnetron start-up current surge is maintained at a minimum.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principlesof the invention may be employed.

In the drawing:

FIG. 1 is a schematic electric circuit diagram of an energization circuit for energizing a magnetron; and

FIG. 2 is a motor, cam, and switching contacts arrangement for use in the circuit of FIG. 1.

Referring now to the drawing wherein like reference numerals refer to like elements in the several figures, an energization circuit for an electrical apparatus, such as, for example, a magnetron or other electric apparatus for operation to obtain selective average output power levels, is shown generally at 10 in FIG. 1. The energization circuit includes an interlock circuit portion 11 coupled across a pair of input terminals 12 for supplying power, for example, from the power company, a start circuit 13 for providing starting power without large current surges, an operation selection portion 14 for selecting cook or defrost circuit configurations, a multiple stage cycling circuit 15 for use in the defrost circuit configuration, and an output circuit 16 for energizing the magnetron 17. The output circuit 16 includes a coupling transformer 18 having a primary winding 19 coupled to the energization circuit 10 and a first secondary winding 20 providing heater energization in the magnetron 17 and a circuit 21 including a second secondary winding providing high voltage energization for the magnetron.

A fuse 22 is connected to one input terminal 12 to provide power to what will be referred to hereinafter as the power side of the energization circuit 10, and the other input terminal 12 is coupled to the neutral line thereof, such terminology being for convenience only since the circuit is preferably coupled for receipt of AC power. The fuse 22 is connected to a main power switch 23, and an oven light 24, stirrer motor 25, and fan motor 26 are coupled between the power and neutral sides of the energization circuit for energization upon closure of the main power switch. The oven light 24 is positioned, for example, to illuminate the interior of the microwave oven cooking cavity, and the stirrer motor 25 and fan motor 26 drive conventional elements used in microwave ovens. If desired, the power switch 23 may be mechanically ganged by a linkage 27 with other switches in the energization circuit 10 as will be described in more detail below.

The interlock circuit portion 11 of the energization circuit 10 includes a latch switch 30, thermostat switch 31 responsive to magnetron temperature, door switch 32, and a timer operated switch 33, the latter having, for example, a closed set position with an open re-set position controlled by a conventional timer 34. The interlock circuit portion 11 is arranged in the energization circuit to prevent energization of the magnetron 17 unless the microwave oven door is closed and latched, the temperature of the magnetron is below that at which the thermostat switch 31 is set, and the timer operated switch 33 is in the set position, not yet having been open by the timer 34. Such interlock portion arrangement assures safe operation of the microwave oven, whereby the magnetron 17 cannot be operated unless the oven cooking cavity is closed and sealed to prevent radiation leakage therefrom.

The start circuit 13 of the energization circuit includes a series coupled current limiting resistor 35 and start switch 36, the latter being, for example, a normally open push button switch. A normally open relay operated switch 37 is coupled in parallel with the current limiting resistor 35 and start switch 36 and is closed by energization of the relay 38, which is connected between the node 39, representing the connection point of the start switch 36 and the relay operated switch 37 remote from the power input terminal 12, and the neutral side of the circuit.

A selector switch 40 in the operation selection portion 14 has a movable switch blade operable for connection to a first contact 41 to couple the energization circuit 10 in configuration for operation in the cooking mode and a second contact 42 to couple the energization circuit in configuration for operation in the defrost mode. When the selector switch 40 is connected to the contact 41, a direct low impedance circuit provides maximum current energization of the magnetron 17 to produce a substantially constant microwave energy level in the oven cooking cavity for cooking the food therein.

When the selector switch 40 is connected to the contact 42 in the defrost position, a low impedance drive motor 43 is connected between the node 39 and the neutral side of the energization circuit. The drive motor 43 is mechanically coupled to a plurality of normally open contacts 44, 45, 46 forming the multiple stage cycling switch to provide periodically a closed circuit of selected impedance to the transformer 18 for energization of the magnetron 17. The contacts 44, 46 are connected to opposite sides of a current limiting resistor 47, which limits current flow to the transformer 18 when the magnetron 17 is initially energized at the beginning of each operational cycle thereof in the defrost mode.

A cook pilot light 48 is connected to the node 39 and through the low impedance drive motor 43 to the neutral side of the energization circuit, and a defrost pilot light 49 is coupled between the contact 42 of the selector switch 40 and the neutral side of the energization circuit, the pilot lights being coupled in the energization circuit for respective energization without requiring added switching elements. When the selector switch 40 is in cook position and the energization circuit is operating, a current flows from the node 39 through the cook pilot light 48 and the low impedance drive motor 43 to neutral to effect indication of circuit energization in the cook mode. The high resistance of the cook pilot light limits the current flow through the drive motor 43 to a level insufficient to energize the same. When the selector switch 40 is in the defrost position, the cook pilot light 48 is short circuited, and a circuit for energizing the low impedance drive motor 43 and the defrost pilot light 49 is established to provide an indication that the energization circuit 10 is operative in the defrost mode.

Referring now specifically to FIG. 2, one type of multiple stage cycling switch 15 is illustrated. Such switch 15 provides a plurality of selected impedances between the terminals 39, 50, including zero and infinite impedance as well as well as an impedance equal to the value of the current limiting resistor 47. In the multiple stage cycling switch the low impedance drive motor 43 drives a cam member 51 connected to the drive shaft thereof, and the cam member includes a plurality of protrusions 52, 53 for operating the contacts 44 through 46. The contact 45 of FIG. 1 is shown in FIG. 2 as two contacts 45a, 45b located on a cantilever member 54, and the contact 46 is located on a similar cantilever member 55, whereby as the cam member 51 is rotated by the low impedance drive motor 43, a connection is first made between the contacts 45b and 46 and later between the contacts 45a and 44. When the connection between the contacts 45b, 46 is made, current from the node terminal 39 must flow through the current limiting resistor 37 to reach the terminal 50 and the transformer 18, whereas when the connection between the contacts 45a and 44 is made, the current limiting resistor is bypassed, and maximum current is provided to the transformer. As the cam member 51 is driven further, the several contacts 44 through 46 are opened, and no power is delivered to the transformer 18.

The energization circuit 10 provides operating potential to the magnetron 17 for energizing the same either continuously in the cook mode or cyclically in the defrost mode, whereby in the latter case the average power output of the magnetron is reduced from maximum to facilitate the defrosting of food in a microwave oven without rapidly cooking the same. In operation of the energization circuit 10, the power switch 23 is closed to provide power to the oven light 24 and the stirrer and fan motors 25, 26 for their conventional purposes. When the selector switch 40 is placed in the cook position and the microwave oven door is closed and latched and the timer operated switch 33 set, a closure of the start switch 36 provides current through the current limiting resistor 35 to the primary winding of the transformer 18 to energize the magnetron 17, the resistor protecting the magnetron and energization circuit from large start-up current surges. Moreover, a small current flows from the node 39 through the relay 38 to close the relay operated switch 37 bypassing the resistor 35 and start switch 36.

Some of the current at the node 39 also energizes the timer 34 and the cook pilot light 48 to indicate that the energization circuit 10 and magnetron 17 are operational in the cook mode. The current flowing through the pilot light 48 passes to the neutral line through the low impedance drive motor 43, such current, however, being insufficient to energize the drive motor at this time. When the microwave oven door is unlatched or opened, the temperature in the microwave oven cooking cavity exceeds that at which the thermostat 31 is set, the time set on the timer 34 expires, or the power switch 23 is opened, then the energization circuit 10 is de-energized, and the magnetron ceases to produce output microwave energy. In normal operation when the time duration for operation set on the timer 34 expires, the timer operated switch 33 is re-set to open the energization circuit causing de-energization of the relay 38 which opens the relay operated switch 37, and the circuit can only be re-energized as described above.

For operation in the defrost mode, the selector switch 40 is placed in the defrost position, and the power switch 23 and switches through 33 are set as above. A closure of the start switch 36 permits current flow through the current limiting resistor to the node 39 from which some-current flows to the timer 34 and the relay 38 for operating the same as above. Also, some of the current from the node 39 passes to the low impedance drive motor 43 for driving the same and to the defrost pilot light 49, which indicates that the energization circuit and magnetron are connected for operation in the defrost mode. As the low impedance drive motor 43 rotates, a first power circuit is provided from the node 39 through the contacts 44 and 45 and the current limiting resistor 47 to the transformer 18 for initially energizing the magnetron 17, the resistor avoiding high start-up surge currents. As the low impedance drive motor 43 rotates further, a connection between the contacts 45 and 46 is made bypassing the current limiting resistor 47, and full power is provided to the magnetron 17. Moreover, when the contacts 44 through 46 are open, for example, as the cam member 51 rotates to a neutral position, the magnetron 17 is deenergized. In one embodiment defrost operation has been effective when using a one minute cycle in which the magnetron is energized for approximately thirty seconds of the cycle with current flowing through the current limiting resistor for about one-half second each time the magnetron is energized. Thus, the current limiting resistor may be of the low power type since current flow therethrough occurs only briefly in each cycle.

The above-described cyclical operation of brief low power energization, full power energization, and off time effects an average energy output from the magnetron 17 suitable for performing defrost operation in a microwave oven while preventing start-up current surges as the magnetron is initially energized in each cycle. Thus, the life of the magnetron and the energization circuit is increased, and the tolerances of the safety fuse or circuit breaker 22 can be more limited to provide increased safety for the overall apparatus.

While the multiple stage cycling switch 15 is illustrated and described as a drive motor with a cam having one or more protrusions for operating a plurality of contact members, other types of switches or switching arrangements may be used, such as, for example, two single pole single throw make before break switches, bimetal switches, double-upper" switches, solid state switches, etc. The main criteria, however, is to assure that a current limiting resistor is connected in series with the transformer 18 whenever the magnetron is initially energized. The contacts 44 through 46 may be opened together, in reverse sequence, or in the same sequence as closed since the reverse inductive current produced when the contacts are opened is relatively small. Also, if desired, the power switch 23, start switch 36, and selector switch 40 may be ganged as shown by the mechanical coupling 27 whereby a closure of the start switch also closes the power switch and an opening of the power switch also assures that the selector switch is in the cook position.

Thus. it should now be understood that applicant has provided an energization circuit for providing power to an electric apparatus, for example, a magnetron, to energize the same to produce output energy at selected average levels. The energization circuit includes current limiting means for protecting the electric apparatus as well as the energization circuit from high current surges during initial start-up or energization of the electric apparatus when used either in continuous operation or cyclical operation.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a highfrequency energy generator, comprising means for connecting the energization circuit to a source of electric energy, means for coupling energy in the energization circuit to the high-frequency energy generator for energization of the same, selector switch means for coupling the energization circuit in a first configuration to provide a cooking function and in a second configuration to provide a defrost function, and cyclical switch means coupled to said selector switch means and energized thereby when the energization circuit is coupled in the defrost configuration for cyclically providing current to said means for coupling for effecting cyclical energization of the high-frequency energy generator, said cyclical switch means comprising current limiting means for limiting the current to said means for coupling, and said cyclical switch means upon energization by said selector switch means being operable in cyclical manner for providing current to said means for coupling via said current limiting means, then bypassing said current limiting means to provide maximum current to said means for coupling, and then interrupting current to said means for coupling, whereby at the initiation of each energization cycle of the high-frequency generator said current limiting means protects the same from current surges.

2. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 1, said cyclical switch means comprising means for bypassing said current limiting means after the beginning of each energization cycle of the generator.

3. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 2, further comprising a plurality of interlock switch means connected in the energization circuit for providing current flow therethrough only when the microwave oven door is closed and latched.

4. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a highfrequency energy generator as set forth in claim 3, further comprising start switch means for providing starting current in the energization circuit for effecting energization of the high-frequency energy generator, said start switch means comprising a current limiting resistor and a normally open switch coupled in series circuit relation, a relay operated switch connected in parallel with such series circuit, and a relay for operating said relay operated switch, said relay being connected to said start switch means for energization upon actuation of the latter.

5. An energization circuit for a microwave oven in which cooking and defrosting functions are accoml '7 plished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 2,

wherein said cyclical switch means comprises electrical switch means for cyclically providing to the means for coupling an open circuit, a circuit including said means for limiting the current, and a closed circuit.

6. An energization circuit for a microwave oven in which cooking and defrosting functions are accomswitch means. 

1. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high-frequency energy generator, comprising means for connecting the energization circuit to a source of electric energy, means for coupling energy in the energization circuit to the high-frequency energy generator for energization of the same, selector switch means for coupling the energization circuit in a first configuration to provide a cooking function and in a second configuration to provide a defrost function, and cyclical switch means coupled to said selector switch means and energized thereby when the energization circuit is coupled in the defrost configuration for cyclically providing current to said means for coupling for effecting cyclical energization of the high-frequency energy generator, saiD cyclical switch means comprising current limiting means for limiting the current to said means for coupling, and said cyclical switch means upon energization by said selector switch means being operable in cyclical manner for providing current to said means for coupling via said current limiting means, then bypassing said current limiting means to provide maximum current to said means for coupling, and then interrupting current to said means for coupling, whereby at the initiation of each energization cycle of the high-frequency generator said current limiting means protects the same from current surges.
 2. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 1, said cyclical switch means comprising means for bypassing said current limiting means after the beginning of each energization cycle of the generator.
 3. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 2, further comprising a plurality of interlock switch means connected in the energization circuit for providing current flow therethrough only when the microwave oven door is closed and latched.
 4. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high-frequency energy generator as set forth in claim 3, further comprising start switch means for providing starting current in the energization circuit for effecting energization of the high-frequency energy generator, said start switch means comprising a current limiting resistor and a normally open switch coupled in series circuit relation, a relay operated switch connected in parallel with such series circuit, and a relay for operating said relay operated switch, said relay being connected to said start switch means for energization upon actuation of the latter.
 5. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by high-frequency energy emitted by a high frequency energy generator as set forth in claim 2, wherein said cyclical switch means comprises electrical switch means for cyclically providing to the means for coupling an open circuit, a circuit including said means for limiting the current, and a closed circuit.
 6. An energization circuit for a microwave oven in which cooking and defrosting functions are accomplished by a high-frequency energy emitted by a high frequency energy generator as set forth in claim 5, wherein said cyclical switch means further comprises a motor and a cam means mechanically connected thereto for controlling operation of said last mentioned switch means. 